The present application claims priority to Japanese Application(s) No(s). P2000-338728 filed Nov. 7, 2000, which application(s) is/are incorporated herein by reference to the extent permitted by law.
The present invention relates to a fuel cell and a fuel cell system including the fuel cells, and particularly to a fuel cell having a function as a small-sized secondary cell and a fuel cell system including the fuel cells.
Fuel cells are known of a type in which a fuel electrode is connected to an oxygen electrode via an ion exchange membrane. In the fuel cell of this type, the ion exchange membrane has a proton conductor for conducting hydrogen ions, that is, protons generated at the fuel electrode to the oxygen electrode. As the proton conductor, there is used a solid polymer composed of a proton conductive organic material such as perfluorosulfonic acid resin (for example, sold by Du Pont Kabushiki Kaisha under the trade name of Nafion). The solid polymer composed of an organic material can conduct protons under a wet condition.
In a related art fuel cell, a fuel such as hydrogen gas is supplied from a fuel supply apparatus, which is provided outside the fuel cell, to a fuel electrode in order to generate hydrogen ions, that is, protons in the fuel electrode. On the other hand, moisture is supplied from a humidifier to an ion exchange membrane for bringing the ion exchange membrane into a proton conductive state.
The above-described related art fuel cell, however, has a disadvantage that since electric power is generated by supplying a fuel such as hydrogen gas from the fuel supply apparatus to the fuel electrode, it is difficult to use the fuel cell as a small-sized chargeable secondary cell applied to portable electronic devices or the like.
To be used as a secondary cell, a fuel cell may be configured by applying a negative voltage and a positive voltage to a fuel electrode and an oxygen electrode, respectively, bringing the oxygen electrode into contact with water, to generate protons, electrons, and oxygen at the oxygen electrode and generate hydrogen from the protons and the electrons at the fuel electrode, storing the hydrogen thus generated, and generating power by using the hydrogen thus stored. Such a fuel cell, however, has the following problem: namely, even if hydrogen can be stored, it is required yet to provide a humidifier for bringing an ion exchange membrane into a wet state, with a result that it is impossible to use the fuel cell as a small-sized secondary cell such as a button type cell.
An object of the present invention is to provide a fuel cell usable as a small-sized secondary cell such as a button type cell, and a fuel cell system including the fuel cells.
To achieve the above object, according to a first aspect of the present invention, there is provided a fuel cell including: a first electrode having a catalyst for generating hydrogen; a second electrode having a catalyst for generating oxygen, the second electrode being provided while allowed to be in contact with water; and a proton conductive electrolyte membrane having a proton conductor produced by introducing proton dissociative groups into a base body composed of a carbonaceous material containing carbon as a main component, the electrolyte membrane being provided between the first electrode and second electrode; wherein when a negative voltage is applied to the first electrode and a positive voltage is applied to the second electrode, oxygen, protons and electrons are generated from water under the presence of the catalyst at the second electrode, and hydrogen is generated from the protons and the electrons under the presence of the catalyst at the first electrode. Here, the wording xe2x80x9cdissociation of protonsxe2x80x9d means a phenomenon that protons (H+) are dissociated by ionization, and the wording xe2x80x9cproton dissociative groupxe2x80x9d means a function group from which hydrogen ions (protons, H+) are dissociated by ionization.
With this configuration, it is possible to generate hydrogen under the presence of water.
The fuel cell, preferably, further includes a storing material for capturing and storing the hydrogen generated at the first electrode.
With this configuration, since the fuel cell has the storing material, it can store hydrogen generated under the presence of water in the storing material, and thereby perform so-called charging.
In the fuel cell, preferably, the first electrode functions, in a state that no voltage is applied to the first electrode, as a fuel electrode which comes in contact with the hydrogen stored in the storing material, to generate protons and electrons from the hydrogen under the presence of the catalyst at the first electrode; the electrolyte membrane functions, in a state that no voltage is not applied to each of the first electrode and second electrode, as an ion exchange membrane which conducts the protons generated at the first electrode to the second electrode; and the second electrode functions, in a state that no voltage is applied to the second electrode, as an oxygen electrode which comes in contact oxygen, to generate water from the oxygen, the electrons, and the protons under the presence of the catalyst at the second electrode; whereby the fuel cell releases electric power as a whole, to thus perform power generation.
With this configuration, when it is not required to generate power, so-called charging can be performed by generating hydrogen under the presence of water, and when it is required to generate power, power generation can be performed by using the generated hydrogen, with a result that the fuel cell can be used like an ordinary secondary cell.
The storing material is preferably made from fullerene molecules, carbon nanotubes, or carbon nanofibers.
With this configuration, so-called charging can be easily and highly densely performed.
The storing material is preferably made from a hydrogen storing alloy.
With this configuration, so-called charging can be easily and highly densely performed.
In the fuel cell, preferably, a separation membrane for preventing the storing material from being corroded is provided between the storing material and the first electrode.
With this configuration, even if the first electrode is made from a material which may corrode the storing material, the storing material can be prevented from being corroded by the separation membrane.
The separation membrane is preferably a hydrogen selectively permeable membrane.
With this configuration, the separation membrane allows selective permeation of only hydrogen.
The separation membrane is preferably made from polyethylene, polypropylene, or polytetrafluoroethylene
With this configuration, the permeation ability of hydrogen in the separation membrane can be enhanced.
Preferably, the storing material is in the form of fine particles which are aggregated into a storing body, and the storing body is disposed in proximity to the first electrode or directly connected to the first electrode.
With this configuration, since the storing material is in the form of fine particles, a total contact area of the storing material with hydrogen, which is capable of capturing and storing the generated hydrogen, can be enlarged, and since the storing body is disposed in proximity to the first electrode or directly connected to the first electrode, the fuel cell can be miniaturized to the level of a small-sized secondary cell such as a button type cell.
In the fuel cell, preferably, a separation membrane for preventing the fine particles of the storing material from being scattered to the first electrode is provided between the storing material and the first electrode.
With this configuration, it is possible to prevent fine particles of the storing material from being scattered to the first electrode.
The separation membrane is preferably a hydrogen selectively permeable membrane.
With this configuration, the fuel cell allows selective permeation of only hydrogen.
The separation membrane is preferably made from polyethylene, polypropylene, or polytetrafluoroethylene.
With this configuration, the permeation ability of hydrogen in the separation membrane can be enhanced.
To achieve the above object, according to a second aspect of the present invention, there is provided a fuel cell system having a plurality of membrane-electrode assemblies. Each of the membrane-electrode assemblies includes: a first electrode having a catalyst for generating hydrogen; a second electrode having a catalyst for generating oxygen, the second electrode being provided while allowed to be in contact with water; and a proton conductive electrolyte membrane having a proton conductor produced by introducing proton dissociative groups into a base body comprising a carbonaceous material containing carbon as a main component, the electrolyte membrane being provided between the first electrode and second electrode; wherein when a negative voltage is applied to the first electrode and a positive voltage is applied to the second electrode, oxygen, protons and electrons are generated from water in the presence of the catalyst at the second electrode, and hydrogen is generated from the protons and the electrons in the presence of the catalyst at the first electrode; and wherein the first electrode functions, in a state that no voltage is applied to the first electrode, as a fuel electrode which comes in contact with the hydrogen, to generate protons and electrons from the hydrogen in the presence of the catalyst at the first electrode; the electrolyte membrane functions, in a state that no voltage is not applied to each of the first electrode and second electrode, as an ion exchange membrane which conducts the protons generated at the first electrode to the second electrode; and the second electrode functions, in a state that no voltage is applied to the second electrode, as an oxygen electrode which comes in contact oxygen, to generate water from the oxygen, the electrons, and the protons in the presence of the catalyst at the second electrode; whereby the membrane-electrode assembly acts as a fuel cell, to release electric power, thus performing power generation.
With this configuration, so-called charging by generating hydrogen under the presence of water and power generation by releasing electric power can be separately, independently performed by individual membrane-electrode assemblies.
Each of the membrane-electrode assemblies preferably has a storing material for capturing and storing the generated hydrogen and supplying the hydrogen to the fuel electrode.
With this configuration, when it is not required to generate power, so-called charging can be performed by generating hydrogen under the presence of water and thus generated hydrogen is stored in a storing material, and when it is required to generate power, power generation can be performed by using the generated hydrogen. Further, since a large amount of hydrogen can be simultaneously generated and a large amount of electric power can be simultaneously generated by using a plurality of the membrane-electrode assemblies, the fuel cell system becomes advantageous when it is required to generate a large amount of hydrogen for a short time for achieving charging for a short time or to generate a large amount of electric power at the time of power generation.
In the fuel cell system, preferably, at least one of the membrane-electrode assemblies acts as a gas supply source for generating hydrogen, and at least one of the rest of the membrane-electrode assemblies acts as a power generator communicated to the gas supply source.
With this configuration, since the hydrogen generated at one membrane-electrode assembly can be used for power generation at another membrane-electrode assembly, a specification of one membrane-electrode assembly can be determined as being optimum to hydrogen generation and a specification of another membrane-electrode assembly can be determined as being optimum to power generation, with a result that it is possible to efficiently perform hydrogen generation and power generation by making specifications of the membrane-electrode assemblies different from each other.